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The stably stratified nocturnal boundary layer (SBL) can be classified into two distinct regimes: one with moderate to strong winds, weak stratification and mechanically sustained turbulence (wSBL) and the other one with moderate to weak wind conditions, strong stratification and collapsed turbulence (vSBL). With the help of a hidden Markov model (HMM) analysis of the three dimensional state variable space of stratification, mean wind speeds, and wind shear the SBL can be classified in these two regimes in both the Reynolds-averaged as well as turbulence state variables. The two-regime SBL is a generic structure at different tower sites around the world independent of the location specific conditions. Besides clustering the data the HMM analysis calculates the most likely regime occupation sequence which allows for detailed analysis of the structure of the meteorological state variables in conditions of very persistent nights. Conditioning on these very persistent nights clear influences of external drivers (such as pressure gradient force and low level cloud cover) are found. As the HMM analysis captures regime transitions accurately changes of state variables and external drivers across transitions can easily be assessed. Different meteorological state variables behave in times of turbulence collapse (wSBL to vSBL transition) and turbulence recovery (vSBL to wSBL transitions) as expected physically. The results reveal further that clear precursors for transitions in the state variable profiles or external drivers are cannot be determined and that on observed timescales regime transitions are relatively sharp. The absence of clear precursors suggests that parameterisations of SBL regime behaviour and turbulence in the two regimes in weather and climate models have to be stochastic. As regime statistics are relatively insensitive to changes in the stochastic properties of the HMM analysis observed regime statistics are compared to ’freely-running’ Markov chains. The SBL regime statistics do not follow a simple Markov process and more complex parameterisations are necessary. A possible approach of parameterising SBL regime behaviour stochastically using climatological results from this analysis is presented. / Graduate / 2019-12-17

abstract: Structural features of canonical wall-bounded turbulent flows are described using several techniques, including proper orthogonal decomposition (POD). The canonical wall-bounded turbulent flows of channels, pipes, and flat-plate boundary layers include physics important to a wide variety of practical fluid flows with a minimum of geometric complications. Yet, significant questions remain for their turbulent motions' form, organization to compose very long motions, and relationship to vortical structures. POD extracts highly energetic structures from flow fields and is one tool to further understand the turbulence physics. A variety of direct numerical simulations provide velocity fields suitable for detailed analysis. Since POD modes require significant interpretation, this study begins with wall-normal, one-dimensional POD for a set of turbulent channel flows. Important features of the modes and their scaling are interpreted in light of flow physics, also leading to a method of synthesizing one-dimensional POD modes. Properties of a pipe flow simulation are then studied via several methods. The presence of very long streamwise motions is assessed using a number of statistical quantities, including energy spectra, which are compared to experiments. Further properties of energy spectra, including their relation to fictitious forces associated with mean Reynolds stress, are considered in depth. After reviewing salient features of turbulent structures previously observed in relevant experiments, structures in the pipe flow are examined in greater detail. A variety of methods reveal organization patterns of structures in instantaneous fields and their associated vortical structures. Properties of POD modes for a boundary layer flow are considered. Finally, very wide modes that occur when computing POD modes in all three canonical flows are compared. The results demonstrate that POD extracts structures relevant to characterizing wall-bounded turbulent flows. However, significant care is necessary in interpreting POD results, for which modes can be categorized according to their self-similarity. Additional analysis techniques reveal the organization of smaller motions in characteristic patterns to compose very long motions in pipe flows. The very large scale motions are observed to contribute large fractions of turbulent kinetic energy and Reynolds stress. The associated vortical structures possess characteristics of hairpins, but are commonly distorted from pristine hairpin geometries. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2012

[pt] O problema tratado nesta dissertação é o da modelagem da excitação, propagação e sensoriamento de ondas acústicas de alta frequência em compósitos piezoelétricos laminados. Empregou-se uma formulação de vigas baseada na teoria discreta de Reddy para compósitos laminados. Esta teoria tem como característica o emprego de funções de interpolação de ordem arbitrária para descrever a variação dos campos de deslocamento e do potencial elétrico ao longo da espessura do laminado. A utilização deste tipo de teoria possibilita a representação de campos de deslocamento que variam rapidamente ao longo da espessura, e que na faixa de altas frequências, são representativos daqueles associados com ondas guiadas em compósitos laminados. No domínio da frequência, o sistema de equações que descreve a resposta eletromecânica do laminado piezoelétrico é colocado na forma de uma equação de estado. A técnica de solução utilizada é baseada no método da imersão invariante, também conhecido como método da varredura ou método de Riccati. Para avaliar os limites da faixa de frequências onde diferentes aproximações são válidas, empregou-se uma técnica baseada na comparação entre o espectro de dispersão exato e aquele previsto pela teoria aproximada. Os resultados destas comparações, mostram que utilizando-se o número de funções de interpolação apropriado, a teoria discreta de Reddy é capaz de representar ondas de comprimento até quatro vezes menores que a espessura da viga. Na faixa de baixas frequências, os resultados são comparados com soluções numéricas fornecidas pelo método dos elementos finitos. Também são apresentados resultados que mostram a variação na resposta eletro-mecânica da viga quando descontinuidades são introduzidas nesta, bem como resultados de experimentos que envolvem a utilização de piezoelétricos como atuadores e sensores. / [en] The proposed contribution adresses the problem of modeling high- frequency excitation, propagation, and sensing, of structural waves in composites containing piezoelectric sensors and actuators. The model for the active composite beam proposed here is based on Reddy’s discrete layerwise laminate theory. The displacement and electric potential are interpolated in the through-thickness direction using piecewise linear functions. In the frequency domain, the governing equations are written in a state space form. Comparisons between approximate and exact wave dispersion spectra are performed in order to assess the efficiency of the layerwise theory in a given frequency range. It is shown that by employing the proper interpolation, Reddy s theory is able to describe the dynamic response of the composite at frequencies where the associated wavelengths are of the same order or even shorter than the thickness of the piezoelectric layers. Next, the solution for the state space equation is obtained by employing an algorithm based on a discrete version of the Riccati transformation, It is shown that the algorithm is stable over a wide range of frequencies. In the low frequency range, the analytical results are compared with finite element results. Also are shown the change in the electrical and mechanical response when discontinuites are introduced in the beam, as well as basic experiments involving piezoelectric sensors and actuators.

A construção do campo pictórico: acúmulos e sobreposições baseia-se na pesquisa sobre uma prática plástica, na construção de um campo pictórico. Assim, é apresentada, nesta dissertação, uma série de pinturas, produzidas entre 2002 e 2004, que tiveram como conceitos norteadores as noções de acúmulos e sobreposições exploradas na elaboração de pinturas. A narrativa é construída em paralelo ao processo de pintar. Seguindo, preferencialmente, a mesma seqüência de construção, procura-se apresentar os elementos que dão condição à instauração desses trabalhos. A partir da análise dessa experiência, se propõe o levantamento de questões recorrentes nessa prática, desde a seleção de referenciais, passando pelas ações operatórias, bem como elementos de naturezas variadas que envolvem uma prática plástica particular e que encontram reverberações na produção. Assim, são abordados assuntos como: o uso do recurso fotográfico na pintura, a exploração da matéria pictórica, a importância da ação do tempo nesse processo, os registros de trabalho e os reflexos e os diálogos firmados com a obra de outros pintores. Essa pesquisa estrutura-se como uma documentação de um processo de trabalho. Nesse sentido, aqui são apresentadas as hesitações, os questionamentos, as hipóteses e as experimentações relativas ao pintar. / The construction of the pictorial field: accumulations and overlaps is based on research about the plastic practice, in the construction of a pictorial field. Hence, it is presented in this dissertation a series of paintings, produced between 2002 and 2004, which had as guiding concepts the notions of accumulations and overlaps explored in the elaboration of paintings. The narrative is built in parallel to the process of painting. Proceeding, preferentially, the same construction sequence, trying to present the elements that allow the instauration of these works. Starting from the analysis of this experience, one intends the rising of appealing subjects in such practice, from reference selection, considering the operative actions, as well as elements varied in nature that involve a particular plastic practice and that find reverberations in the production. Hence, the approached subjects are: the use of the photographic resources in painting, the exploration of the pictorial matter, the importance of time effect in this process, the work recordings and the reflexes and dialogues with the other painters' work. This research is structured as a documentation of a work process. In that sense, here, the hesitations, the questionings, the hypotheses and the experimentations related to painting are presented.

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A construção do campo pictórico: acúmulos e sobreposições baseia-se na pesquisa sobre uma prática plástica, na construção de um campo pictórico. Assim, é apresentada, nesta dissertação, uma série de pinturas, produzidas entre 2002 e 2004, que tiveram como conceitos norteadores as noções de acúmulos e sobreposições exploradas na elaboração de pinturas. A narrativa é construída em paralelo ao processo de pintar. Seguindo, preferencialmente, a mesma seqüência de construção, procura-se apresentar os elementos que dão condição à instauração desses trabalhos. A partir da análise dessa experiência, se propõe o levantamento de questões recorrentes nessa prática, desde a seleção de referenciais, passando pelas ações operatórias, bem como elementos de naturezas variadas que envolvem uma prática plástica particular e que encontram reverberações na produção. Assim, são abordados assuntos como: o uso do recurso fotográfico na pintura, a exploração da matéria pictórica, a importância da ação do tempo nesse processo, os registros de trabalho e os reflexos e os diálogos firmados com a obra de outros pintores. Essa pesquisa estrutura-se como uma documentação de um processo de trabalho. Nesse sentido, aqui são apresentadas as hesitações, os questionamentos, as hipóteses e as experimentações relativas ao pintar. / The construction of the pictorial field: accumulations and overlaps is based on research about the plastic practice, in the construction of a pictorial field. Hence, it is presented in this dissertation a series of paintings, produced between 2002 and 2004, which had as guiding concepts the notions of accumulations and overlaps explored in the elaboration of paintings. The narrative is built in parallel to the process of painting. Proceeding, preferentially, the same construction sequence, trying to present the elements that allow the instauration of these works. Starting from the analysis of this experience, one intends the rising of appealing subjects in such practice, from reference selection, considering the operative actions, as well as elements varied in nature that involve a particular plastic practice and that find reverberations in the production. Hence, the approached subjects are: the use of the photographic resources in painting, the exploration of the pictorial matter, the importance of time effect in this process, the work recordings and the reflexes and dialogues with the other painters' work. This research is structured as a documentation of a work process. In that sense, here, the hesitations, the questionings, the hypotheses and the experimentations related to painting are presented.

A construção do campo pictórico: acúmulos e sobreposições baseia-se na pesquisa sobre uma prática plástica, na construção de um campo pictórico. Assim, é apresentada, nesta dissertação, uma série de pinturas, produzidas entre 2002 e 2004, que tiveram como conceitos norteadores as noções de acúmulos e sobreposições exploradas na elaboração de pinturas. A narrativa é construída em paralelo ao processo de pintar. Seguindo, preferencialmente, a mesma seqüência de construção, procura-se apresentar os elementos que dão condição à instauração desses trabalhos. A partir da análise dessa experiência, se propõe o levantamento de questões recorrentes nessa prática, desde a seleção de referenciais, passando pelas ações operatórias, bem como elementos de naturezas variadas que envolvem uma prática plástica particular e que encontram reverberações na produção. Assim, são abordados assuntos como: o uso do recurso fotográfico na pintura, a exploração da matéria pictórica, a importância da ação do tempo nesse processo, os registros de trabalho e os reflexos e os diálogos firmados com a obra de outros pintores. Essa pesquisa estrutura-se como uma documentação de um processo de trabalho. Nesse sentido, aqui são apresentadas as hesitações, os questionamentos, as hipóteses e as experimentações relativas ao pintar. / The construction of the pictorial field: accumulations and overlaps is based on research about the plastic practice, in the construction of a pictorial field. Hence, it is presented in this dissertation a series of paintings, produced between 2002 and 2004, which had as guiding concepts the notions of accumulations and overlaps explored in the elaboration of paintings. The narrative is built in parallel to the process of painting. Proceeding, preferentially, the same construction sequence, trying to present the elements that allow the instauration of these works. Starting from the analysis of this experience, one intends the rising of appealing subjects in such practice, from reference selection, considering the operative actions, as well as elements varied in nature that involve a particular plastic practice and that find reverberations in the production. Hence, the approached subjects are: the use of the photographic resources in painting, the exploration of the pictorial matter, the importance of time effect in this process, the work recordings and the reflexes and dialogues with the other painters' work. This research is structured as a documentation of a work process. In that sense, here, the hesitations, the questionings, the hypotheses and the experimentations related to painting are presented.